426 Singletube damper

A single-tube damper is shown in Fig. 42.3. The spaces immediately above and below the piston are filled with oil, and the damping action arises from the viscous losses that occur in the orifices, as in the double-tube type. However the effect of the volume of the piston rod is allowed for by the use of a volume of gas under compression at the bottom of the tube, where it is separated from the oil by a floating piston. When the tube of the damper moves upwards, the gas will be further compressed and the floating piston will be moved downwards relative to it by the amount required to accommodate the changes in the volume of the two oil spaces. This compression of the gas results in a progressive change in the characteristic of the damping, such that the force required to move the damper tube upward at a constant speed will rise at an increasing rate. In contrast, in the double-tube type, the rate of increase is constant.

In a single-tube damper, the pressure of the gas - usually nitrogen - must be higher than the maximum operating pressure in the fluid below the main piston, and may be of the order 2.5 MN/m2. This of course adds to the total spring-rate of the suspension by an amount equal to the gas pressure multiplied by the effective cross-sectional area of the piston rod.

The Woodhead Manufacturing Company has produced a single-tube damper without a floating piston. In it the inert gas is free in the cylinder and therefore tends to emulsify the fluid. Although the gas and oil separate while the vehicle is stationary, re-emulsification occurs rapidly owing to the large flow rates inherent in the design, the performance of the unit therefore becomes normal equally rapidly. An advantage of this type of damper is the impossibility of jacking up and subsequent bending of the piston rod, in the event of leakage of fluid past a free piston. Also, again because of the absence of the

free piston, the dead length of the damper is small. Moreover, the performance of these emulsion-type dampers is affected less than that of the fluid type by variations in working temperature. Single-tube dampers in general have the advantage that, for a given overall diameter, the piston area is greater, but they have the disadvantage of a higher built-in pressure.

Telescopic dampres are also used in the MacPherson, or strut-type, suspension, Fig. 43.10. Here, a major problem arises because of the imposition of side loads at the top of the strut, and therefore bending moments on the piston rod. Because such a piston rod, taking these loads and performing guidance function, has to be of large diameter, the problems arising owing to its intrusion into the damper are aggravated.

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